(Reuters) - Israel's cabinet authorized the mobilization of up to 75,000 reservists late on Friday, preparing the ground for a possible Gaza invasion after Palestinians fired a rocket toward Jerusalem for the first time in decades.

Tel Aviv, Israel's commercial centre, also came under rocket attack for the second straight day, in defiance of an Israeli air offensive that began on Wednesday with the declared aim of deterring Hamas from launching cross-border attacks that have plagued southern Israel for years.

Hamas, the Islamist group that runs the Gaza Strip, claimed responsibility for firing at Jerusalem and Tel Aviv. Israel said the rocket launched toward Jerusalem landed in the occupied West Bank, and the one fired at Tel Aviv did not hit the city. There were no reports of casualties.

The siren that sounded in Jerusalem stunned many Israelis. The city, holy to Jews, Muslims and Christians, was last struck by a Palestinian rocket in 1970, and it was not a target when Saddam Hussein's Iraq fired missiles at Israel in the 1991 Gulf War.